Side girl pregnancy isn't in the game yet simply because there's just so darn many of them :P I will put as many as possible in, but I may have to run a poll over on Patreon/Subscribestar at some point and pick the top ten or something...
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I would like to recommend leaving it as a "post game" bonus. That way you won't have to jump through hoops doing that while also having the myriad other things in the works lol. Then that leaves it open for you to add in fun(ny) quirks for whichever side character, and it won't possibly clash with anything you do later on.